1. Field of the Invention
The object of this invention is to provide a hard wear resistant aluminum oxide coating of kappa-Al.sub.2 O.sub.3 resulting in a grade suitable for turning and milling of steels with clearly improved wear properties.
2. Description of the Prior Art
Al.sub.2 O.sub.3 coatings can be deposited on cemented carbides applying an intermediate coating consisting of TiC, Ti(C,N), TiN or a combination of these. Alternatively the substrate may be .gamma.-phase enriched. On top of the alumina coating a thin layer of TiN is often deposited, partly to give the tool an attractive appearance.
In addition to the alpha-Al.sub.2 O.sub.3 usually referred to as corundum, alumina also exhibits several metastable allotropic modifications. These are gamma, delta, eta, theta, kappa and chi (Stumpf 1930, Thibon 1951). Only alpha exhibits an clearly established crystal structure consisting of close-packed planes of oxygen ions stacked in hcp sequence (ABAB . . . ) the aluminum cations occupying two thirds of the octahedral interstices (Kronberg 1957).
The metastable alumina phases can be classified in two major groups: The alpha series, with an hcp stacking of anions like corundum, and the gamma series, with fcc oxygen stacking of a spinel type (Levi 1988). The alpha series includes chi and kappa while the gamma series consists of eta, gamma, delta and theta. In wear resistant CVD alumina coatings, the only metastable polymorph of interest--in addition to stable alpha--is kappa. The oxygen arrangement of kappa is possibly ABAC . . . type of close packing, aluminum cations being obviously also here situated in octahedral interstices. The unit cell dimensions are a=9.60 A and c=9.02 A and density 3.67 g/cm.sup.3 (Okumiya et al 1971). At a higher temperature (T - 1000.degree. C.) kappa--like all metastable alumina phases--transforms into alpha, which is the equilibrium crystal structure of alumina. For kappa this transformation most probably includes both a rearrangement of aluminum cations and a large-scale rearrangement of the ABAC . . . stacking of the oxygen sublattice into the ABAB . . . stacking of oxygen in corundum (alpha alumina).
In CVD coatings, the alpha and kappa forms of alumina can often be found simultaneously since they have only slight differences in thermochemical stabilities and it is not always straightforward to nucleate and deposit coatings of pure .alpha.-Al.sub.2 O.sub.3 and especially pure kappa-Al.sub.2 O.sub.3. The deposition of kappa-Al.sub.2 O.sub.3 is further complicated by the kappa.fwdarw.alpha transformation which may occur during the CVD-process and by the fact that the nucleation of Al.sub.2 O.sub.3 in the alpha modification becomes more dominant with increasing oxide coating thickness (assuming that Al.sub.2 O.sub.3 has nucleated in kappa modification).
Only a few reports on the microstructure of CVD deposited alpha and kappa alumina phases can be found in the literature. It is, however, well established that .alpha.-Al.sub.2 O.sub.3 exhibits a considerably larger inherent grain size than kappa-Al.sub.2 O.sub.3. Grains of alpha alumina are typically equiaxed and can grow through the whole coating layer resulting in a very large grain size of the order of 3-6 .mu.m depending on the total alumina coating thickness. This is especially pronounced when deposition is carried out at atmospheric or high pressure (500-1000 mbar). Reduced deposition pressure (about 100 mbar) results in the grain refinement of alpha alumina with a common grain size of .alpha.-Al.sub.2 O.sub.3 formed under reduced pressure being of the order 1-2 .mu.m. Typical crystallographic defects found in alpha alumina are dislocations and voids which occur in abundance. Linkage of voids at grain boundaries occur resulting into the formation of long channels of voids (Vuorinen 1984, 85). This obviously increases the brittleness of the alpha alumina coating. The kappa-Al.sub.2 O.sub.3 coating has a grain size of the order of 0.5-1 .mu.m and often exhibits a pronounced columnar coating morphology. In a kappa-Al.sub.2 O.sub.3 coating dislocations and especially voids are almost totally absent (Vuorinen and Skogsmo 1988).
It is relatively uncomplicated to nucleate and deposit coatings of .alpha.-Al.sub.2 O.sub.3 at atmospheric pressures. As already mentioned above the resulting .alpha.-Al.sub.2 O.sub.3 coating exhibits, however, relatively very large grain size (usually clearly faceted grains) and consequently pronounced uneven coatings are obtained. These coatings are thus morphologically not very suitable for wear resistant applications. Further, the coating thickness has to be limited to 3-5 .mu.m for the reasons mentioned above.
Kappa alumina has been claimed to exhibit good wear properties in some applications. The deposition of pure kappa-Al.sub.2 O.sub.3 is, however, feasible only to limited coating thicknesses (2-3 .mu.m). In this regard the following two complications should be considered. First, in thicker coatings even though originally nucleated as kappa-Al.sub.2 O.sub.3, the kappa.fwdarw..alpha. transformation might occur as a result of the longer deposition time (annealing). This often results in severe cracking of the alumina layer due to the fact that the alpha modification is more dense (3.99 g/cm.sup.3) than the kappa modification (3.67 g/cm.sup.3). The volume contraction being consequently of the order of about 8% upon transformation.
Second, as mentioned earlier, nucleation of .alpha.-Al.sub.2 O.sub.3 becomes clearly more favorable when the thickness of the oxide layer exceeds 2-3 .mu.m. Consequently simultaneous nucleation and growth of alpha and kappa Al.sub.2 O.sub.3 occur. In this case, due to differences in grain size (growth rate) of alpha and kappa modifications the coexistence of these oxides in the coating layer may result in the formation of very uneven oxide wear layers. In both cases discussed above, the boundary regions between alpha and kappa phases will constitute regions of considerable mechanical weakness.
Various prior art cutting tools (Hale's U.S. Pat. Nos. 32,110 and 4,608,098 and Lindstrom U.S. Pat. No. 31,520) employ .alpha.-Al.sub.2 O.sub.3 coatings deposited on either .gamma.-phase enriched cemented carbides or TiC-coated cemented carbides employing also one or several bonding layers between the substrate and the .alpha.-Al.sub.2 O.sub.3 layer. While products made according to these patents reflect substantial strides over the prior art products and have enjoyed considerable commercial success, a need still arises for improvement in these techniques. For example, the deposition of .alpha.-Al.sub.2 O.sub.3 performed according to these patents at atmospheric pressures results in the uneven coating structures above unless other compounds are added to the CVD gas mixture (Smith U.S. Pat. No. 4,180,400). Furthermore these coatings usually require at least a two-stage coating process. In U.S. Pat. No. 4,180,400 a deposition process for coating consisting mainly of kappa-phase is disclosed. This process is, however, sensitive to the kappa.fwdarw.alpha transformation and further nucleation of kappa-Al.sub.2 O.sub.3 cannot be ensured at the original nucleation surface (which is usually a TiC coated cemented carbide). The coatings produced according to this invention are kappa-Al.sub.2 O.sub.3.
A method for forming an aluminum oxide coating of sufficient thickness (diffusion wear resistance), acceptable coating morphology and excellent adhesion to the underlying substrate, produced economically with simple means, has not yet been proposed.
The object of the present invention described in detail below is to provide a novel method meeting such a demand, the resulting product and a method of using that product.